There are a number of procedures and devices by which one can observe the geometry of surfaces in general.
One of these procedures, particularly suitable for applications in the automotive and aeronautic industries, consists of using an optical device with a camera directed toward the surface to be observed, this camera being connected to a computer and generating an image in shades of gray, showing variations in the surface's curvature. Devices of this kind are marketed, for example, under the trademarks DIFFRACTO® and ONDULO®, and are generally used for optically reflective surfaces, permitting qualitative readings by means of images in shades of gray. In this way, these devices can be used to visualize slight variations in slope or curvature by using the principle of specular reflection, but they do not permit quantitative readings of geometrical magnitudes of these variations. Therefore, it is impossible to know the value of the slopes or curvatures of the optically reflective surfaces.
Other known procedures make it possible to measure geometric magnitudes of these optically reflective surfaces.
A first procedure consists of using a mechanical feeler that moves in relation to the optically reflective surface to be measured. This mechanical feeler is calibrated so that one or more geometric magnitudes of the surface can be made to correspond to each displacement of its measuring finger. At the time of measurement, the finger of the mechanical feeler generally follows a predetermined path to sweep the entire surface. In this way, it is possible to measure the geometric magnitude(s) of the path followed according to the axis of the mechanical feeler. The greatest disadvantages of this procedure are the time needed to measure the surface, the lack of precision resulting from the limited number of measuring points, and finally the risks of surface damage from contact with the finger of the mechanical feeler.
A second known procedure uses the techniques of interferometry. This procedure compares two waves from the same laser, one reflected by the surface to be measured, and the other a reference coming directly from the laser. Like the previous procedure, the latter is suitable for measuring small surfaces. However, for measuring large surfaces such as mirrors, windshields, and others, this process is particularly expensive and time-consuming. In addition, this process is appropriate only for perfect optically reflective surfaces, which does not describe the panels of automobile bodies, for example.
Therefore, existing procedures do not permit a rapid and precise measurement of large optically reflective surfaces.